The present invention relates to the fabrication of integrated circuit devices, and more particularly, to methods of forming silicon nitride layers on silicon-comprising substrate surfaces, methods of forming capacitors, and to capacitor constructions.
Silicon nitride (Si3N4) layers are frequently used in the fabrication of semiconductor wafers, for example, in the fabrication of MOSFET gates, memory cells, and precision capacitors. Silicon nitride layers are utilized as an insulation layer over silicon surfaces to electrically isolate conductive components of a semiconductor circuit from one another. Silicon nitride films are also used as a diffusion barrier to protect regions of a semiconductor wafer during local oxidation of silicon due to the slow speed with which oxygen and water vapor diffuse in the nitride. Silicon nitride dielectric films are preferred over silicon dioxide (SiO2) in many applications due to its high dielectric constant (6-9 versus about 4.2 for chemical vapor deposited (CVD) SiO2).
Capacitors are commonly-used electrical components of semiconductor circuitry. A typical capacitor is constructed of two conductive plates separated by a non-conducting dielectric layer. The capacitor is electrically connected to a circuit external of the capacitor. The dielectric layer is preferably comprised of one or more materials having a very high dielectric constant and low leakage current characteristics, for example, SiO2 and Si3N4, with Si3N4 being typically preferred due to its higher dielectric constant.
Silicon nitride is typically deposited upon a silicon surface (including, but not limited to, single crystal, poly, and epitaxial), by low pressure chemical vapor deposition (LPCVD). In a conventional silicon nitride LPCVD deposition, the silicon surface is normally pretreated, for example, by removing the native oxide using a hydrofluoric acid (HF) clean solution. A film of silicon nitride is then deposited on the pretreated surface by LPCVD by reacting dichlorosilane (SiH2Cl2) and ammonia (NH3) over the silicon surface in a hot-wall reactor at about 600 to about 800xc2x0 C. and a pressure of about 100 mTorr to about 2 Torr.
An alternate process for producing a Si3N4 layer is by rapid thermal nitridation (RTN) of a silicon layer which is annealed in an NH3 or other nitrogen-comprising atmosphere, which reacts with the silicon to produce silicon nitride. However, the growth of the nitride is extremely slow and self-limiting since the NH3 is not capable of adequately diffusing through the growing silicon nitride layer to react with the underlying silicon. The ultimate thickness of a silicon nitride film produced by nitridation is typically only 3 to 4 nm at high temperatures. Such thickness is usually too low to adequately function as a barrier to prevent further oxidation of the silicon surface during subsequent processing, or as a capacitor dielectric layer between two conductive capacitor plates. Further, the electrical quality of this high temperature nitridated layer is poor.
Another described technique for formation of Si3N4 capacitor dielectric layers is to initially nitridize an outer silicon surface to obtain a 20 to 30 angstrom thick layer, and then deposit a silicon nitride film by low pressure chemical vapor deposition of DCS. The drawbacks of these methods include poor electrical performance of the RTN/DCS layer and high thermal budget induced from the high temperature RTN process.
Surface properties of the wafer surface play an important role in the initial growth of films in thin-film processes, which will impact the properties and structure of the thin film that is deposited. Different nucleation and deposition rates occur for the deposition of silicon nitride on different wafer surfaces. This leads to different or degraded electrical characteristics of semiconductor devices having different wafer surfaces that are fabricated using a silicon nitride deposited layer. In addition, deposition of silicon nitride also includes an incubation time at the start of the deposition where there is no apparent deposition of silicon nitride. The incubation time may extend up to several minutes for some surfaces. Surfaces exhibiting such different rates and incubation times include, but are not limited to, one or more of tetraethylorthosilicate (TEOS), borophosphosilicate glass (BPSG), silicon, polysilicon, hemispherical grain (HSG) polysilicon, other doped silicon or polysilicon surfaces, other doped oxides, thermal silicon dioxide, chemical vapor deposited (CVD) silicon dioxide, and plasma enhanced CVD (PECVD) silicon dioxide.
TCS nitride layers (SiCL4 reached with NH3) have been shown to have much better capacitance (Cp)-leakage performance than silicon nitride layers produced by dichlorosilane (DCS) in DRAM capacity modules. However, there have been problems with nucleation of TCS nitride on certain silicon-based surfaces such as borophosphosilicate glass (BPSG) using current TCS cell nitride processes.
An example of a prior art process for depositing a TCS silicon nitride layer in a DRAM capacity module is described with reference to FIGS. 1A-1B. Referring to FIG. 1A, an exemplary semiconductor wafer fragment 10, shown in a preliminary processing step, comprises a base layer 12, an insulative layer 14 of BPGS formed over base layer 12, an HSG polysilicon layer 16 overlying the BPSG insulative layer 14, and an opening 18. As shown in FIG. 1B, the HSG polysilicon layer 16 is incorporated into a capacitor construction 20 in which a silicon nitride-comprising dielectric layer 22 comprising a TCS nitride layer 24 and a capacitor plate layer 26, are formed over the HSG polysilicon layer 16. In a conventional process, the TCS nitride layer 24 can be deposited over both the BPSG layer 14 and the HSG polysilicon layer 16 by LPCVD by reacting silicon tetrachloride (SiCl4) and ammonia (NH3) over the substrate in a hot-wall reactor at about 500 to about 800xc2x0 C. and a pressure of about 100 mTorr to about 2 Torr.
As depicted in FIG. 1B, transmission electron microscopy (TEM) cross-sectional images have shown that portion xe2x80x9cAxe2x80x9d of the TCS nitride layer 24 deposited on the surface of the BPSG substrate 14 to be comparatively thinner (about 15 to about 20 angstroms) than the thickness of portion xe2x80x9cBxe2x80x9d of the TCS nitride layer 24 deposited on the HSG polysilicon substrate 18 (about 45 to about 50 angstroms).
At a thickness of less than about 20 angstroms, the nitride is not able to prevent the polysilicon electrode from becoming oxidated in the wet re-oxidation step that follows the cell nitride deposition. Consequently, re-oxidation results in the oxidation of the bottom polysilicon electrode. The volume expansion from the polysilicon oxidation will force the BPSG container sidewall to xe2x80x9cbulge outxe2x80x9d of the wafer surface. These bulges appear as xe2x80x9cbubblingxe2x80x9d from a top view in a Scanning Electron Microscopy (SEM) image. This is commonly referred to as xe2x80x9cBPSG bubblingxe2x80x9d and the nitride is said to be xe2x80x9cpunched throughxe2x80x9d. These bubblings are fatal physical defects and will result in massive fail of the part. Beside preventing the BPSG bubbling problem, a desirable thickness of nitride on the BPSG also functions as a H2 barrier in the subsequent wet re-oxidation and other processes to protect the active area in the device because H2 could enhance boron diffusion and deactivate the transistor. The poor nucleation of TCS nitride on BPSG has limited the use of TCS nitride to below about 50 angstroms in current DRAM capacity modules.
Therefore, it is desirable to develop an improved process of forming dielectric silicon nitride films with TCS that will overcome the shortcomings of present TCS nitride processes. It is also desirable to provide an improved process to enable the production of a silicon nitride layer using TCS that will be of adequate thickness over substantially all silicon substrates of a device to provide the desired capacitor dielectric properties in a capacitor construction, and improve the characteristics of the fabricated semiconductors devices.
The present invention provides an improved method for forming silicon nitride films on semiconductor devices.
The present invention combines depositing a DCS nitride seeding layer with depositing a TCS nitride layer on a silicon-comprising substrate to improve the thickness of a dielectric silicon nitride layer deposited using TCS on silicon-comprising substrates of a semiconductor device being constructed. Before the deposition of the TCS nitride, a thin layer of DCS nitride is deposited as an interface xe2x80x9cseeding layerxe2x80x9d on the surface of the silicon-comprising substrate for the formation of the TCS nitride layer such as by low pressure chemical vapor deposition (LPCVD). Advantageously, the DCS seeding layer can be deposited without a conventionally used NH3 pre-anneal nitridation step, by utilizing a pressure of about 1-3 Torr in the reaction chamber.
In one embodiment of the method of the invention, a silicon nitride layer is formed on a surface of a silicon-comprising substrate by first depositing a DCS nitride seeding layer on the surface to a thickness of up to about 5 angstroms, and then depositing a TCS silicon nitride layer on the DSC silicon nitride layer to a desired thickness for the application.
In another embodiment, the invention provides a method of forming a silicon nitride layer on a surface of a silicon-comprising substrate by first nitridizing the surface by exposure to a nitrogen-comprising gas of at least one of N2, NH3 or NOx, preferably NH3, to form at least a monolayer of silicon nitride to a thickness of less than about 5 angstroms. A DCS silicon nitride seeding layer is then deposited on the nitridated surface to a thickness of up to about 5 angstroms, and a TCS silicon nitride layer is then deposited on the DSC silicon nitride seeding layer to a desired thickness, typically about 40-50 angstroms or less.
In another method of the present invention, a silicon nitride layer is formed on a surface of a silicon-comprising substrate that includes two or more discrete areas comprised of different silicon-comprising materials, for example, TEOS, BPSG, polysilicon, HSG polysilicon, among others. A silicon nitride layer is formed on the surface of the various silicon-comprising substrates by depositing a thin DCS silicon nitride seeding layer on the surface to a thickness of up to about 5 angstroms, and then depositing a TCS silicon nitride layer on the DSC silicon nitride layer to a desired thickness. The cross-wafer thickness of the TCS silicon nitride layer is substantially equivalent over the entire substrate surface, regardless of the silicon-comprising substrate.
In yet another method of the invention, the surface of two or more discrete areas of different silicon-comprising substrates is nitridized to form at least a monolayer of silicon nitride thereon, and a silicon nitride layer is deposited on the surface of the nitridated layer. The silicon nitride layer is formed by depositing a DCS silicon nitride seeding layer on the nitridated layer, and then depositing a TCS silicon nitride layer on the DSC silicon nitride layer. The cross-wafer thickness of the resulting silicon nitride layer is substantially equivalent from area to area over the surface of the substrate.
In another aspect, the invention encompasses a method of forming a capacitor. The method includes forming a first capacitor plate comprising a silicon-comprising substrate, for example, HSG polysilicon, forming a dielectric layer of silicon nitride proximate the first capacitor plate, and forming a second capacitor plate over the silicon nitride layer. The silicon nitride layer is formed by first depositing a thin DCS silicon nitride seeding layer of up to about 5 angstroms on the surface of the first capacitor plate, and then depositing a TCS silicon nitride layer on the DSC silicon nitride seeding layer, to a desired thickness, for example, about 40-50 angstroms. Preferably, the DCS and TCS silicon nitride layers are deposited by chemical vapor deposition, preferably by LPCVD.
In another method of forming a capacitor according to the invention, a capacitor is prepared by forming a first capacitor plate comprising a silicon-comprising substrate surface such as HSG polysilicon, a silicon nitride dielectric layer proximate the first capacitor plate, and a second capacitor plate over the dielectric layer. The silicon-based substrate surface of the first capacitor plate is nitridized in a nitrogen-comprising ambient, preferably NH3, to form from one to three monolayers of silicon nitride on the substrate surface, preferably up to about 2 angstroms. A thin DCS silicon nitride seeding layer of about to about 5 angstroms is then deposited on the nitridated silicon-based surface, and a TCS nitride layer is deposited on the DSC nitride layer to the desired thickness.
In another aspect, the invention provides a capacitor. The capacitor includes a first capacitor plate comprising a silicon-comprising substrate, a second capacitor plate, and a dielectric silicon nitride layer intermediate the first and second capacitor plates. In a first embodiment of a capacitor according to the invention, the silicon nitride layer is composed of a thin DCS silicon nitride seeding layer up to about 5 angstroms deposited on the first capacitor plate, and a TCS silicon nitride layer deposited on the DSC silicon nitride layer to a desired thickness.
In another embodiment, the capacitor includes a silicon nitride layer intermediate first and second capacitor plates, the silicon nitride layer composed of a nitridized layer of silicon nitride on the surface of the first capacitor plate, preferably up to about 2 angstroms thick or from three to less than one monolayers of silicon nitride, a thin DCS nitride seeding layer of up to about 5 angstroms deposited on the nitridated layer, and a TCS nitride layer deposited on the DSC silicon nitride layer to a desired thickness.
Semiconductor devices fabricated with the silicon nitride deposition method according to the present invention to provide a DCS/TCS nitride layer, have substantially the same electrical performance as a conventional device fabricated with a silicon nitride layer made of TCS nitride alone, and improved Cp-leakage performance over a device made with a DCS nitride layer alone. Also, with the use of the DCS seeding layer, the nucleation and deposition rate of the TCS nitride component upon the substrate is substantially equivalent regardless of the silicon-comprising material that constitutes the substrate. This alleviates the problem of different or degraded electrical characteristics that result from the differences in the nitride thickness deposited on adjacent surfaces. Such differences are especially apparent, for example, between a conductor composed of silicon, and an insulator composed of TEOS, where a relatively thin nitride layer deposited at a silicon/TEOS edge can result in degraded electrical properties at that edge and for the resulting fabricated device.
The use of the DCS/TCS nitride layer also resolves the fatal BPSG bubbling problem that occurs with TCS nitride layers that are deposited directly on BPSG substrates in semiconductor devices. Additionally, the elimination of a high temperature NH3 pre-anneal step from the process conserves the thermal budget for other processes in the fabrication operation.